A heat-stable inhibitor of the phosphodiesterase that is activated by the calcium-dependent regulatory protein (CDR) has been partially purified from bovine brain. This material decreases cGMP phosphodiesterase activity whether or not Ca ions are present; it shifts to the right the dose-response curve for activation by Ca ions plus CDR. It is noncompetitive with regard to the substrate cGMP at concentrations below 2 micromoles. It does not inhibit a cGMP PDE from rat liver that is not activated by Ca ions-CDR. Studies with CDR affinity columns suggest that the inhibitor binds to CDR in a Ca ions-dependent process.